1. Field of Invention
The present invention is related to a control circuit, especially to a control circuit with ZVS-lock and asymmetrical PWM for resonant power converter.
2. Description of Related Art
For achieving higher efficiency, a switching frequency of the resonant power converter should be closely to the resonant frequency of the resonant power converter during a heavy load and/or the low input voltage conditions. In other words, the switching frequency of the resonant power converter should be at the ZVS (zero voltage switching) regions (regions 1 and 2 shown in FIG. 2) during the heavy load and/or the low input voltage conditions. However, when the switching frequency is decreased in response to the increase of the load and/or the decrease of the input voltage of the resonant power converter, the switching frequency might fall into the ZCS (zero current switching) region (region 3 shown in FIG. 2). The control of the resonant power converter would become a none-linear operation if the switching frequency is decreased to fall into the region 3. Besides, the resonant power converter is operated at higher switching frequency during the light load, which will increase the switching loss and result poor efficiency. The description of the resonant power converter and asymmetrical PWM operation for the resonant power converter can be found in the prior arts of “Switching controller for resonant power converter”, U.S. Pat. No. 7,313,004; “ASYMMETRICAL RESONANT POWER CONVERTERS”, U.S. patent application N.O. 2010/0202162.
Therefore, the present invention is developed to prevent that the resonant power converter is operated in region 3 and ensure the ZVS (zero voltage switching) operation for heavy load. Furthermore, the present invention develops a method that allows the resonant power converter operated at the PWM mode with ZVS for light load to achieve power saving.